Pakistan, India and Israel are the only three countries that have never signed the Nuclear Non-Proliferation Treaty, a pact that promises nuclear cooperation in exchange for a pledge to forgo nuclear weapons. All three of those states have nuclear weapons, though Israel has not declared its nuclear arsenal.

During a hearing before the House Foreign Affairs Committee, Vann H. Van Diepen, the acting assistant secretary of state for international security and nonproliferation, said the United States will vote against any exemption for China to sell two civil nuclear reactors to Pakistan.

In response to a question from Rep. Ed Royce, California Republican, asking whether the United States would vote against the exemption for China, Mr. Van Diepen said, “Yes sir, by definition, we do not support any activity that goes against the guidelines.”

Earlier, Mr. Van Diepen said, “Based on the facts we are aware of, it would occur to us that this sale would not be allowed to occur without an exemption of the NSG.”

However, Mr. Van Diepen added that while the United States can vote against an exemption, it cannot stop China if that nation decides to sell Pakistan the reactors without special permission from the NSG.

In 2008, India and the United States signed a nuclear cooperation agreement that would give India access to reactors and other nuclear technology without having to sign the Nuclear Non-Proliferation Treaty. Pakistan and India are archrivals.

Earlier this week in Pakistan, Mrs. Clinton told Pakistani journalists, “In our dialogue with the Pakistani government, we have clearly said we will work with them on civil nuclear energy.”

She added, “It took years to do it with India. But we are committed to pursuing it and trying to overcome the obstacles that might stand in the way, because we think it is important to get as much of a varied source of energy all connected to the grid.”

China last spring signed a $2.4 billion agreement with Pakistan to supply two 340-megawatt reactors to Pakistan.

Chinese Foreign Ministry spokesman Qin Gang told reporters in Beijing June 24 that China’s nuclear cooperation with Pakistan is for peaceful purposes and in line with international obligations of both countries.

China in the past was a major supplier of nuclear technology to Pakistan, and U.S. intelligence officials said its bomb design is modeled after China’s nuclear weapons.

A senior State Department official said, “Pakistan has profound energy needs, and we are working with Pakistan to try to increase its energy production and the diversity of its energy resources.”

The official said, “In the abstract, nuclear power at some point can be part of that mix; in the near to midterm, we are focused on nonnuclear sources of energy.”

The official said the United States is “willing to have a conversation about civilian nuclear power, but there is a lot that Pakistan is going to have to do, given its past record” - a reference to the covert nuclear supplier group led by Pakistani scientist A.Q. Khan that supplied nuclear bomb designs and material to Iran, Libya and Pakistan.

Henry Sokolski, director of the Nonproliferation Policy Education Center, said it is not clear when and how the United States would open the door to nuclear exports to Pakistan.

“Is Washington blocking China so the Obama administration can be in control of the terms under which it will allow a Chinese sale to Pakistan?” Mr. Sokolski asked. “Or is it that only the United States wants to be the one making the reactor sale? I think it’s the former; the mystery is what we will ask for in exchange.”

In the hearing, Mr. Van Dieppen also said Chinese entities continue to sell arms to rogue states in violation of anti-arms-proliferation controls.

He said that while China has made some progress in establishing arms-export controls, Chinese companies continue to engage in dangerous sales to countries they should not.

“Our most persistent problem is that individual Chinese entities continue to engage in proliferation activities,” he said.